According to some sources, the test certificate (this 'Page 40' document) is from testing of Merlin III improvements - of what has later become the Merlin XII. Unfortunately, without the rest of this paperwork (previous 39 pages) we can only assume what exactly is that all about I am afraid...
Anyway, knowing RR habit of testing and 'breaking' engines while improving parts that fail first to get more power, this makes perfect sense to me. Ratings of both Merlin XII and Merlin III are well documented and researched - Merlin in perspective pg. 155 and onwards states all ratings according to improvements along from III to XII:
R.M.1.S. (Merlin III)
combat power: 1310hp, 3000rpm, 9000ft., +12lbs
(combat power: 1440hp, 3000rpm, 5500ft., +16lbs - Sea Hurricane only)
take off: 880hp, 3000rpm, +6.25lbs.
cruising: 2600rpm, +4.5lbs.
climbing: 2600rpm, +6.5lbs.
R.M.2.S. (Merlin III, as R.M.1.S. but with increased take-off power on 100 octane fuel, superseded by the use of combat ratings on R.M.1.S.)
combat power: 1000hp, 3000rpm, 15500ft.,+6.25lbs.
take off: 1000hp, 3000rpm, +8.25lbs.
cruising: 2600rpm, +4.5lbs.
climbing: 2600rpm, +6.5lbs.
R.M.3.S. (as Merlin IV but higher supercharger gear ratio (note was 8.588 on Merlin III, is 9.089 on Merlin XII) and 100 octane fuel. For Spitfire II)
combat power: 1280hp, 3000rpm, 10500ft.,+12lbs.
take off: 1175hp, 3000rpm, +12.5lbs.
cruising: 2650rpm, +7bs.
climbing: 2850rpm, +9lbs.
R.M.4.S. was regarding further improvements on Merlin XII, but was never production type; superseded by Merlin 45
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